Postal blue indicia (i.e. markings) on mailing envelopes has been used as evidence of postage payment in many countries around the world, including France, Germany, Finland, Austria, Switzerland, Belgium, and Spain. Several other countries including UK and China are also migrating to postal blue indicia. These postage indicia contain machine-readable information, e.g. two-dimensional (2-D) bar codes, that are often printed by inkjet printing at high speed. The 2-D bar codes enable automated sorting of mails because a large amount of information can be recorded at high density. In some examples, in order for the printed indicia to be read effectively by postal machines such as optical scanners and 2-D bar code readers, the printed indicia is a high-contrast image. As such, there is imposed a strict high-contrast requirement by many countries. It may be easy for an ink to comply with the postal indicia specification of one country, but it is more difficult for one ink to meet the postal requirements of many or all countries.